All living tridacnines (Tridacna and Hippopus) have zooxanthellae and also filter feed. Having two sources of food may contribute to the large size.
Fragines (such as Frgum and Corculum) have zooxanthellae and apparently rely on them exclusively as adults, as they have reduced digestive tracts.
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa